Determine whether photocoagulation therapy will be useful in preserving vision for patients with diabetic retinopathy. Assuming that photocoagulation does have an effect, Study is designed to compare two commonly used treatment techniques (Xeonon and Argon); to determine whether photocoagulation of selected areas of the retina has a demonstrable morphologic effect on the untreated areas of the retina and if there are certain stages of the disease in which treatment is harmful; and to evaluate the efficacy of photocoagulation through simultaneous long-term observation of eyes not exposed to treatment.